This invention relates to smart cards and in particular to multi layered smart cards having a metal foil/film/layer and an antenna structure for receiving or transmitting signals between the smart cards and a card reader.
As shown in FIGS. 1, 2(a), 2(b) and 2C a typical smart card includes an antenna structure coupled (directly or electromagnetically) to a microprocessor or microcomputer (also referred to as a “chip”) also located on or within the card. The antenna structure functions to enable contactless communication between a card reader (also referred to as a “transponder” or “transceiver”) and the microprocessor. That is: (a) signals emitted by the card reader are electromagnetically coupled via the antenna to the chip which receives and processes the signals; and (b) signals processed and emitted by the chip are electromagnetically transmitted via the antenna to the card reader. In the manufacture of certain smart cards it is highly desirable that a metal foil/film/layer be included among the card layers. However, the metal foil/film/layer presents a problem since it functions to attenuate (absorb) the signals transmitted between the card reader and the chip limiting communication or even making it impossible. As an example, it is a requirement that a card reader should be able to read a smart card located at a given distance from the card reader. Known cards with a metal foil/film/layer could not be read reliably at these established minimum required distances.
To overcome the attenuating effect of the metal foil/film/layer, an insulator layer may be formed between the metal film/foil/layer and the antenna structure. A conventional approach is to make the insulator layer very thick to decrease the attenuating effect of the metal foil/layer. However, this is not acceptable where the permissible thickness of the insulator layer is limited. As is known in the art there are numerous requirements which have to be met in the manufacture of cards. Some go to the structural integrity of the cards (e.g., they should not bend, delaminate) and be capable of use for several years and a large number of user cycles. So, the cards need to be formed using numerous layers with various requirements on the thickness and composition of the layers. Thus, it is not satisfactory to just make the insulator layer arbitrarily very thick since such thick layers interfere with other requirements in the manufacture of smart cards.
The problem of manufacturing a reliable smart card is even greater particularly when the card includes a metal layer which interferes with the transmission/reception of signals between the smart card and a card reader.
Thus, a significant problem faced by Applicants related to selecting the thickness of an insulator layer, extending within a prescribed range, which can provide reliable readings of card data by a card reader located at a prescribed distance and at a prescribed frequency of operation. An associated problem was finding a thickness for the insulator layer which provided improved transmission/reception.